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Pier Protection and Warning Systems for Bridges Subject to Ship Collisions
February 11, 1983
PURPOSE To provide guidance, to the Federal Highway Administration's field offices and to project applicants involved with bridges subject to ship collisions.
APPLICABILITY This material, which is not regulatory, summarizes existing guidance and information on the state-of-the-art -of addressing potential hazards to bridges from ship collisions. The increase in the occurrence of ship-bridge collisions during the past 10 years warrants additional emphasis at this time.
/s/ signed by
The increase in the occurrence of ship-bridges collisions during the past 10 years warrants additional emphasis on the need to consider protection for bridge piers as well as the installation of warning systems to alert motorists in the event of a span collapse.
The purpose of this directive is to provide guidance on these subjects to the Federal Highway Administration's (FHWA) field offices and to State and local agencies involved with Federal-aid highway projects which cross navigable writers. This material is not regulatory, but has been developed to provide additional support and emphasis for developing appropriate protective and warning systems.
Project Development Procedures
The consideration of the potential for ship-bridge collisions should be addressed in the early stages of project development so that cost-effective means can be developed for addressing such hazards. Information from the Coast Guard and the waterway users regarding (1) the type and frequency of shipping on the waterway, and (2) special navigational needs or problems concerning the channel in the vicinity of the bridge, can be most helpful in assessing possible hazards during the location and design of the bridge piers, and the evaluation of the need for motorist warning systems.
The warrants for such systems should be based on assessment of the risks and consequences of a ship-bridge collision, taking into consideration (1) the type and frequency of shipping on the waterway; (2) the location and arrangement of the bridge piers in relation to the navigable channel and the resulting vulnerability of the piers to ship collisions; (3) other factors (fog, channel geometrics, wind, river currents, etc.) which may create navigational problems in the vicinity of the bridge; and (4) volume of highway traffic using the bridge.
Motorist Warning Systems
Several bridge failures involving the collapse of a bridge span have occurred in the recent past as a result of the collision of a ship with the bridge. In the immediate aftermath of such an accident, the potential exists for drivers to be unaware of the danger and to drive off the damaged bridge before warning devices and barricades can he erected. This hazard in compounded by the fact that such accidents are likely to occur at night or in periods of poor visibility.
The Department of Transportation has been evaluating the various factors involved in ship-bridge collisions in an effort to find ways to reduce the severity and occurrence of such accidents. While this type of catastrophic failure is not common, enough accidents do occur to warrant consideration of the need for motorist warning systems on bridges subject to ship collisions.
At this time, the most practical warning device is an electrical conductor attached to or a part of the bridge which will activate warning systems and/or gates when the continuity is disrupted (span collapse).
While the use of such a system is simple in concept, there are a number of design considerations (sources of power, need for gates, signals, lights, signs, etc.) to be taken into account. The design and location of warning mechanisms becomes more complex for bridges susceptible to collision over a considerable number of spans. Thus, the warning system must be designed to fit the type of structure, the approaches, and other specific conditions existing at each bridge site.
Appendix A, Alternate Surveillance and Warning Systems for the Sunshine Skyway Bridge Across Tampa Bay, was prepared for the Florida Department of Transportation and contains an excellent comprehensive overview of various factors which should be considered while assessing the need for a motorist warning system. Design Category II, Detection, lists a number of laser, radar, radio and television early warning systems to alert motorists of an impending collision. In general, these systems are still experimental and have not as yet been proven to be reliable for the purpose of providing an early warning to motorists. Accordingly, it is recommended that requests for such systems be submitted to Washington Headquarters for review.
Figure 8 of Appendix A has been modified by the addition of a schematic electrical circuit (Appendix B) in order to illustrate how an electrical circuit (Appendix B) in order to illustrate how an electrical conductor can be installed for purposes of detecting a span collapse. The continuity circuit (a pair of conductor wires) and the switch circuits are normally energized.
In the event of a span collapse, the switch circuits are de-energized, causing the activation of warning signs, alarms or gates by the local power source.
A source of local power must be available at each end of the bridge in order to activate the respective warning systems, and of course, the switches and warning devices should be located beyond the point where damage to the bridge is expected.
Under some circumstances, it is possible that a ship-bridge collision could cause severe displacement of a portion of the bridge without a span collapse so that the conductor would stretch but not break. For this reason, consideration should be given to designing the conductor with a circuit interrupter (mercury switch, bayonet-type pluc, etc.) located at periodic intervals along the bridge so that a severe impact would interrupt the circuit and activate the warning devices (Appendix B).
Because of the tremendous momentum achieved by modern ocean-going vessels even while traveling at low speeds in inland channels, it may be extremely difficult to retrofit some existing bridge piers with protective systems which can successfully withstand the anticipated impact loadings. For this reason, it becomes particularly important to recognize the potential hazards from ship collisions and to locate and design piers on new bridges in such a way that the risks of collision are reduced to an acceptable level.
At this time, FHWA is exploring with the American Association of State Highway and Transportation Officials the feasibility of developing standards for the location and protection of bridge piers in navigable waterways. As a result of a research study commissioned by the Coast Guard, a computer program to analyze impact loadings on piers and protection systems has been developed and is fully operational. This program provides a structural evaluation of the effectiveness of bridge pier protective systems for any selected vessel size, speed and angle of attack. Details of this computer program and further guidance of pier protective systems can be obtained from the Bridge Division.